Flight control actuators typically comprise a housing and a movable part extending from the housing. The housing may be attached to a static part of an aircraft structure while the movable part may be attached to a movable aircraft part such as a flight control surface, an undercarriage component, a thrust reverser cowl, a landing gear, a nose wheel steering mechanism, a tailboard and so on. Typically the housing and the movable actuator part are attached to the respective static and movable aircraft parts via pin, spherical or other connections.
It is frequently desirable to know the forces, both compressive and tensile that are exerted by the actuator on the aircraft. These forces are typically measured using load cells.
In a known arrangement, a load cell is arranged in the load path through the actuator, for example between a component coupled to the movable actuator part and the actuator housing. The load cell is preloaded in compression by an amount which exceeds the maximum tensile stress expected on the actuator in use such that the load cell does not become completely unloaded in use. If a tensile load is applied on the actuator, that load can be calculated by subtracting the measured load from the initial load. If a compressive load is applied that load can be calculated by subtracting the initial load from the measured load.
A potential disadvantage with this arrangement is that the calculation of the actual loads depends upon accurately knowing the preload on the load sensor. The degree of preload may vary over time, for example through stress relief or through temperature changes, which means that the load cell has to be carefully recalibrated at regular intervals. This is not convenient in practice.